refactor: Fix AI billing system - revert to commit #10 + fixes

- Reverted to commit #10 (98e68f6) for stable AI function base
- Fixed database migrations: removed 0018-0019 that broke schema
- Fixed CreditCostConfig schema: restored credits_cost, unit fields
- Fixed historical table schema for django-simple-history
- Added debug system (staged for future use)

Changes:
- CreditCostConfig: Updated OPERATION_TYPE_CHOICES (10 ops, no duplicates)
- CreditUsageLog: Updated choices with legacy aliases marked
- Migration 0018_update_operation_choices: Applied successfully
- All AI operations working (clustering, ideas, content, optimization, etc.)

Test Results:
✓ CreditCostConfig save/load working
✓ Credit check passing for all operations
✓ AICore initialization successful
✓ AIEngine operation mapping functional
✓ Admin panel accessible without 500 errors

Future: AI-MODEL-COST-REFACTOR-PLAN.md created for token-based system

backend/igny8_core/business/system/apps.py

@@ -0,0 +1,11 @@
+"""
+System app configuration
+"""
+from django.apps import AppConfig
+
+
+class SystemConfig(AppConfig):
+    default_auto_field = 'django.db.models.BigAutoField'
+    name = 'igny8_core.business.system'
+    label = 'debug_system'
+    verbose_name = 'Debug & System Settings'